Bio-degradable ‘reef bags’ used for rubble stabilisation and their impact on rubble stability, binding, coral recruitment and fish occupancy

Substrate stabilisation can be used to enhance coral recruitment where mobile rubble beds have formed post-disturbance. We trialled gabion-like ‘reef bags’, biodegradable coconut-fibre coir mesh bags filled with coral rubble at Pinnacle Bay and Bait Reef on the Great Barrier Reef. Most coir remained intact during the first 12 months, but had completely biodegraded after 2 years, leaving behind the experimental rubble mounds. After approximately 2 years, fish abundance was higher above mounds compared to surrounding rubble. After approximately 3 years, rubble stability and binding was also higher in rubble mounds than in surrounding rubble at Pinnacle Bay, but not at Bait Reef (although binding did increase in Bait Reef mounds over this time). The increased stability and binding did not, however, translate to significantly higher coral recruitment on rubble mounds in either location. The placement of these reef bags in terms of depth, reef zone, sediment load and competition appears crucial. Future trials should consider the size and interlocked-ness of rubble, the size of reef bags, larger mesh hole sizes, the number of layers of coir, and include unbagged mounds.